os: clarify windows read console code

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/9458043

src/pkg/os/file_windows.go

@@ -253,12 +253,12 @@ func (f *File) readConsole(b []byte) (n int, err error) {
 	if len(f.readbuf) == 0 {
 		// syscall.ReadConsole seems to fail, if given large buffer.
 		// So limit the buffer to 16000 characters.
-		readN := 16000
-		if len(b) < readN {
-			readN = len(b)
+		numBytes := len(b)
+		if numBytes > 16000 {
+			numBytes = 16000
 		}
 		// get more input data from os
-		wchars := make([]uint16, readN)
+		wchars := make([]uint16, numBytes)
 		var p *uint16
 		if len(b) > 0 {
 			p = &wchars[0]